Re: [PATCH i-g-t 1/2] igt/perf_pmu: Aim for a fixed number of iterations for calibrating accuracy

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 08/08/2018 15:59, Chris Wilson wrote:
> Our observation is that the systematic error is proportional to the
> number of iterations we perform; the suspicion is that it directly
> correlates with the number of sleeps. Reduce the number of iterations,
> to try and keep the error in check.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   tests/perf_pmu.c | 34 +++++++++++++++++++++-------------
>   1 file changed, 21 insertions(+), 13 deletions(-)
> 
> diff --git a/tests/perf_pmu.c b/tests/perf_pmu.c
> index 9a20abb6b..5a26d5272 100644
> --- a/tests/perf_pmu.c
> +++ b/tests/perf_pmu.c
> @@ -1521,14 +1521,13 @@ static void __rearm_spin_batch(igt_spin_t *spin)
>   
>   static void
>   accuracy(int gem_fd, const struct intel_execution_engine2 *e,
> -	 unsigned long target_busy_pct)
> +	 unsigned long target_busy_pct,
> +	 unsigned long target_iters)
>   {
> -	unsigned long busy_us = 10000 - 100 * (1 + abs(50 - target_busy_pct));
> -	unsigned long idle_us = 100 * (busy_us - target_busy_pct *
> -				busy_us / 100) / target_busy_pct;
>   	const unsigned long min_test_us = 1e6;
> -	const unsigned long pwm_calibration_us = min_test_us;
> -	const unsigned long test_us = min_test_us;
> +	unsigned long pwm_calibration_us;
> +	unsigned long test_us;
> +	unsigned long cycle_us, busy_us, idle_us;
>   	double busy_r, expected;
>   	uint64_t val[2];
>   	uint64_t ts[2];
> @@ -1538,18 +1537,27 @@ accuracy(int gem_fd, const struct intel_execution_engine2 *e,
>   	/* Sampling platforms cannot reach the high accuracy criteria. */
>   	igt_require(gem_has_execlists(gem_fd));
>   
> -	while (idle_us < 2500) {
> +	/* Aim for approximately 100 iterations for calibration */
> +	cycle_us = min_test_us / target_iters;
> +	busy_us = cycle_us * target_busy_pct / 100;
> +	idle_us = cycle_us - busy_us;
> +
> +	while (idle_us < 2500 || busy_us < 2500) {
>   		busy_us *= 2;
>   		idle_us *= 2;
>   	}
> +	cycle_us = busy_us + idle_us;
> +	pwm_calibration_us = target_iters * cycle_us / 2;

I'd be tempted not to halve the calibration phase, just to minimize the 
number of changes.

> +	test_us = target_iters * cycle_us;
>   
> -	igt_info("calibration=%lums, test=%lums; ratio=%.2f%% (%luus/%luus)\n",
> -		 pwm_calibration_us / 1000, test_us / 1000,
> -		 (double)busy_us / (busy_us + idle_us) * 100.0,
> +	igt_info("calibration=%lums, test=%lums, cycle=%lums; ratio=%.2f%% (%luus/%luus)\n",
> +		 pwm_calibration_us / 1000, test_us / 1000, cycle_us / 1000,
> +		 (double)busy_us / cycle_us * 100.0,
>   		 busy_us, idle_us);
>   
> -	assert_within_epsilon((double)busy_us / (busy_us + idle_us),
> -				(double)target_busy_pct / 100.0, tolerance);
> +	assert_within_epsilon((double)busy_us / cycle_us,
> +			      (double)target_busy_pct / 100.0,
> +			      tolerance);
>   
>   	igt_assert(pipe(link) == 0);
>   
> @@ -1796,7 +1804,7 @@ igt_main
>   			for (i = 0; i < ARRAY_SIZE(pct); i++) {
>   				igt_subtest_f("busy-accuracy-%u-%s",
>   					      pct[i], e->name)
> -					accuracy(fd, e, pct[i]);
> +					accuracy(fd, e, pct[i], 10);
>   			}
>   
>   			igt_subtest_f("busy-hang-%s", e->name)
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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